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How the Red Planet got two faces

A GIANT asteroid that struck Mars a glancing blow could explain why the planet is lop-sided.

Mars’s northern hemisphere is much flatter and more low-lying than the southern hemisphere. The difference in average elevation between the two is about 5 kilometres. One theory from the 1980s was that a giant asteroid about 300 kilometres across had hit the planet sometime in its early history, causing a permanent depression in the northern hemisphere.

Now, computer simulations by a team led by Margarita Marinova of the California Institute of Technology in Pasadena confirm that a giant asteroid could have done the job, …